Synthetic hydrogels are remarkable platforms for in situ cell delivery in a three-dimensional scaffold where the chemistry and properties can be tuned to promote and facilitate tissue growth. Synthetic hydrogels, however, as with any non-biological material, suffer from the immune system?s normal response to foreign materials. The foreign body response (FBR) is characterized by chronic inflammation and the formation of a dense, avascular fibrous capsule. These events can negatively affect cells embedded within a hydrogel and create a barrier between the scaffold and the host tissue that prevents integration. Given that synthetic hydrogels offer numerous advantages for designing tunable scaffolds (e.g., material stiffness, degradation rates, etc.), a better understanding of the FBR is needed. While macrophages are known to orchestrate the FBR, our current view of macrophages in the FBR is oversimplified. Fundamentally, these has been a recent paradigm shift in how macrophages are viewed. We now know that macrophages from distinctly different origins, ?resident? tissue macrophages and ?recruited? macrophages from blood-derived monocytes, respond to areas of injury, but with distinctly different roles. ?Recruited? monocyte-derived macrophages are thought to be the main drivers of pro- inflammatory and pro-fibrotic responses in injured tissue. Thus, it is reasonable to postulate that these different macrophage populations may be involved in the FBR. The overarching goal of this senior fellowship is to apply this new paradigm to study the macrophage in the context of the FBR. This idea that macrophages, which accumulate at the site of an implant, arrive from different origins has not been considered previously in the FBR. These studies are central to the research training plan that will broaden the PI?s knowledge in immunology, enable her to learn state-of-the-art techniques to study macrophages, and facilitate the PI?s transition to establish a new research direction in immunology of biomaterials for musculoskeletal research. The overarching hypothesis for this project is that ?recruited? macrophages from blood-derived monocytes are primarily responsible for the events that lead to chronic inflammation and fibrosis in the FBR. To test this hypothesis two specific aims are proposed: a) identify the biological effect of macrophages and their functional state on the FBR using CCR2 knockout mouse models and b) identify the temporal biological effect of macrophages and their functional state on the FBR using MaFIA transgenic mouse models. We will employ novel techniques and mouse models that prevent recruitment of monocyte-derived (i.e., ?recruited?) macrophages (i.e., CCR2 knockouts) and that allow us to inducibly deplete all macrophages at discrete time intervals (i.e., MaFIA). The outcome of these studies is to re-define the macrophage in the context of the FBR and importantly identify how different macrophage subpopulations contribute to the FBR. This new insight will be leveraged to design immunomodulatory hydrogels that target the timing and the macrophage subpopulation(s) that is responsible for the FBR.